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Re: Another alternative (Re: format strings are the Right Thing)

This page is part of the web mail archives of SRFI 48 from before July 7th, 2015. The new archives for SRFI 48 contain all messages, not just those from before July 7th, 2015.

A clisp format procedure among others, seems quite reasonable,

<input-port> (fmt-clisp <string> <arg> ...) ; to each his own.

However my gut feel is that format procedures/macros should likely yield
an <input-ports>, as opposed to directly writing to an <output-port>;
as it would seem that being able to produce and accept <input-port>
pipes as arguments would enable the ability to compose relatively
sophisticated efficient format/text-processing hierarchies quite easily,
without requiring the use of potentially inefficient and/or cumbersome
intermediate strings to buffer text between transform layers.


> From: Shiro Kawai <shiro@xxxxxxxx>
> I got an impression that this format string discussion has similarity
> to the regexp pattern language---it defines a mini-language,
> it has long history tons of variations, it is concise in typical
> usage but can't be composed well and tend to produce an
> incomprehensible code when one try to push it too hard.
> And Olin Shivers showed a solution, SRE.
> Can't we do the similar thing here?
> I haven't thought it out well, but the basic idea is like this:
> [macro] sfmt <SFMT-SPEC> ...
> Generates a "formatter" closure.  Which takes output port and list of
> args, and emit the formatted output to the given port.
> <string>                ; literal string to be displayed.
> (write [limit N])       ; take next arg and write it (up to N chars)
> (display [limit N])     ; take next arg and display it (up to N chars)
> (integer [width N][pad C]) ; format as integer
> (float [width N][precision P] ...) ; format as flonum
>  ;; and so on ...
> (cl-formattr <string>)  ; traditinal CL-style format string
> And the ability to insert evaluated expression in SFMT-SPEC will allow
> the programmer to insert other formatter closure into SFMT-SPEC template.
> (But I have an uneasy feeling about the "implicit backquote" feature
> of SRE... it looks like it make difficult to write a macro that
> generates SRE, though  I haven't used SRE extensively so I can't tell
> for sure).
> The legacy format can be defined easily, to support the legacy code:
> [procedure] format output-port string . args
> == ((sfmt (cl-formatter string)) output-port args)
> [procedure] format string . args
> == (let ((out (open-output-string)))
>     ((sfmt (cl-formatter string)) out args)
>     (get-output-string out))
> We could also expose low-level build-in formatter procedure,
> which could be similar to the one Paul showed.
> Issues:
> - What should formatter closure return? - you might want to
>  switch behavior by whether the output is chopped by the length
>  limitation or not, for example.   Also the you need to know
>  how many args are consumed by other formatter procedure embedded
>  in SFMT-SPEC.
> - How to handle reordering of the arguments?  Reordering makes
>  composition of formatters difficult, but is there a better
>  way to support internationalization of formatting templates?
>  (maybe a named argument instead of positional argument?)
> --shiro